operation done for that purpose); 20 percent are using highly effective contraceptives, usually long-term in nature; 5 percent are pregnant or immediately postpartum at any particular point in time; and 11 percent have never had sexual intercourse.

Classification, Anatomy, and Embryology

NTDs are the most common major congenital malformations of the central nervous system. They arise as a result of a disturbance of the embryonic process of neurulation and are midline defects that affect neural tissues, their coverings anywhere along the neuraxis, or both. They are heterogeneous malformations, and the terms used to define them here (see Box 8-1) are based on clinical descriptions and the presumed embryological defect (Lindseth, 1996; Volpe, 1995). The terminology in the literature may vary.

NTDs are not to be confused with spina bifida occulta, a common radiographic finding that does not involve neural elements, or encephalocele, a protrusion of meninges and brain tissue outside the cranium, most frequently in the occipital region.

BOX 8-1 Forms of Neural Tube Defects

Anencephaly: a fatal form characterized by partial absence of brain tissue, presumably caused by failure of closure of the anterior neuropore.

Meningomyelocele: a midline defect of the spinal cord in which the neural tissue is dysplastic and the overlying meninges form a cystic expansion, presumably because of failure of closure of the neural tube at this site. This defect is most often in the lumbosacral region, usually results in peripheral neurological deficit, and may be called spina bifida aperta or cystica. Myelomeningocele often is associated with the Arnold Chiari malformation and hydrocephalus.

Meningocele: a less severe result of the embryological defect that causes meningomyelocele, involving only the meninges.

Craniorachischisis: a fatal form in which the entire neuraxis—from the brain to distal spinal cord—is dysplastic and lacks covering by dura, muscle, or skin. This is presumably due to total failure of neurulation.